The present invention relates to an ultrafine metal powder slurry, and more particularly, relates to an ultrafine metal powder slurry with superior dispersibility, which is used for conductive paste fillers, internal electrodes of multilayer ceramic capacitors, and the like.
An ultrafine metal powder, such as an ultrafine nickel powder, used for internal electrodes of multilayer ceramic capacitors is a powdered high purity metal composed, for example, of particles having an approximately spherical shape and a mean particle diameter of 0.1 to 1.0 μm. An ultrafine metal powder as described above is mixed with a binder such as an organic resin to form a paste for forming the internal electrodes. The paste thus formed is applied onto ceramic green sheets by screen printing or the like to form thin films, followed by lamination of several hundreds of the green sheets thus processed, so that a laminate composite including internal electrode layers is formed. Subsequently, a multilayer ceramic capacitor is formed by processing the above laminate composite through a degreasing step, a sintering step, and a firing step. The mean particle diameter described above indicates a mean volume-surface diameter (d3) in terms of number-size distribution.
Concomitant with the recent trend toward miniaturization and higher capacity of multilayer ceramic capacitors, it has been required that the number of ceramic green sheets including internal electrode layers is increased from several hundreds to approximately one thousand. In order to satisfy this requirement, the thickness of the internal electrode layer is decreased from 3 μm, which has been heretofore used, to 1.5 μm or less.
In addition, when an ultrafine metal powder has poor dispersibility and includes aggregates such as clumps, the aggregates may penetrate a ceramic sheet layer to cause short circuiting of electrodes, and hence defective units are formed. Even when the aggregates do not penetrate a ceramic sheet layer, since the distance between electrodes is decreased, local current crowding occurs, thereby causing degradation and a decrease in the lifetime of a multilayer ceramic capacitor.
Accordingly, the particle size distribution D90 of an ultrafine metal powder used as a raw material for internal electrode layers is preferably decreased as much as possible. The term “particle size distribution (D90)” indicates a particle diameter at a cumulative percentage of 90% (D90) on a volume basis.
In a related production process (Process 20 shown in FIG. 2) of an ultrafine metal powder by a chemical vapor deposition (CVD) method, after residues of a metal chloride used as a raw material for the ultrafine metal powder are removed for purification of the ultrafine metal powder during Step 21, which is a wet washing step for purification, to form a metal-water slurry, the metal powder in the slurry thus obtained is dried during Step 22, and subsequently, an ultrafine metal powder product (dry powder product) is formed during Step 23.
However, during Step 22 of drying the metal powder, since aggregation inevitably occurs due to liquid bridging forces and Van der Waals' forces generated between particles of the metal powder, there is a problem in that the particles are not sufficiently dispersed in an organic solvent during Step 24.
In addition, during Step 22 of drying the metal powder, since metal hydroxides are generated on the surfaces of the particles of the ultrafine metal powder, the ultrafine metal powder cannot have sufficient wettability (lipophilic property) with an organic solvent. As a result, in dispersing the dry powder obtained during Step 23 in an organic solvent during Step 24, there is also a problem in that the particles of the ultrafine metal powder have poor wettability with an organic solvent aggregate with each other.
Hence, although several types of dispersion treatment, such as ball mill dispersion, ultrasonic dispersion, and roll mill dispersion, are performed in combination during Step 25, the particles of the ultrafine metal powder processed by drying during Step 22 are liable to aggregate with each other and have inferior dispersibility. As a result, when the dry powder processed during Step 22 is used, a paste containing approximately up to 50 percent by mass of the ultrafine metal powder is an upper limit obtained by the dispersion treatment.
In general, the dry powder obtained during Step 23 is supplied to customers. Hence, the dry powder obtained during Step 23 is dispersed in an organic solvent (dispersion treatment during Step 24) at a customer site, and subsequently, viscosity adjustment is performed during Step 27 as a final step, thereby forming a paste.
Accordingly, in order to disaggregate aggregated particles generated during Step 22 of drying the metal powder and aggregated particles generated during Step 24 of dispersing the dry powder in an organic solvent, complicated treatment such as dispersion treatment performed in combination with ball mill treatment, ultrasonic treatment, roll mill treatment, and the like during Step 25 and the filtration treatment during Step 26 must be additionally performed. As a result, large amounts of labor and time are required.
Hence, the dry ultrafine metal powder product obtained during Step 23 is required to have superior dispersibility and to contain no aggregated particles.
As a technique related to a dispersion of an ultrafine metal powder capable of satisfying the above requirements, in Japanese Unexamined Patent Application Publication No. 2003-342607, a dispersion of a nickel powder has been disclosed, that is prepared by adding an organic solvent to a dispersion containing a water disperse medium and an ultrafine nickel powder having a mean particle diameter of 1 μm or less so as to replace at least a part of the water disperse medium, and then adding a polar solvent for processing the nickel powder.
According to Japanese Unexamined Patent Application Publication No. 2003-342607, when the dispersion of the nickel powder is prepared, treatment is preferably performed using a carbonated aqueous solution, and by treating the nickel powder in a carbonated aqueous solution, hydroxides present on the surface of the nickel powder by adhesion or adsorption are removed, resulting in further improvement in dispersibility of the nickel powder. In addition, according to the above patent document, the reason for this is believed to be as follows. When hydroxides are present on the surfaces of nickel powder particles by adsorption or the like, since the particles are attracted to each other due to the hydroxyl polarity, the hydrophilic property (suspensibility) of the particles is degraded, and as a result, the nickel powder particles aggregate with each other.
In addition, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 2003-342607, instead of water with an organic solvent, a method has been disclosed in which after a surfactant is added to the dispersion, followed by the addition of the organic solvent. The dispersion thus processed is held still, and the water is then separated by decantation and is further removed by heating at 50 to 150° C. In the above patent document, many types of surfactants are mentioned by way of example, and according to the above disclosed technique, replacement of the water disperse medium with the organic solvent can be easily performed by addition of the surfactant, and superior paste properties can be finally obtained. Furthermore, it has also been disclosed that, in general, a nonionic surfactant having an HLB (hydrophile-lipophile balance) value of 3 to 20 is preferably used.
In addition, as another technique which satisfies the above requirements, an ultrafine metal powder slurry having superior dispersibility was proposed (see Japanese Unexamined Patent Application Publication No. 2004-158397). The ultrafine metal powder slurry is an ultrafine metal powder slurry containing an organic solvent, a surfactant having a hydrophilic group and a lipophilic group, and more than 60 to less than 95 percent by mass of an ultrafine metal powder, in which the hydrophilic group of the surfactant described above is sulfonato group, sulfo group, sulfonyldioxyl group, polyoxyethylene group with carboxyl group, or polyoxyethylene group with phosphate group, and in which the lipophilic group is an alkyl group containing 12 or more carbon atoms or an alkylphenyl group.
In addition, according to Japanese Unexamined Patent Application Publication No. 2003-342607, since the replacement of water with an organic solvent is performed by physical operation using the difference in gravity and by operation removing water using evaporation, for example, decantation operation and drying treatment are required. Accordingly, in particular, as disclosed in Example 1 of Japanese Unexamined Patent Application Publication No. 2003-342607, drying treatment must be performed at 120° C. for 16 hours for 1 kg of a nickel powder, followed by further drying treatment at 100° C. for 48 hours; hence, reduction in labor and reduction in treatment time have been achieved in a conductive paste production process.
On the other hand, according to Japanese Unexamined Patent Application Publication No. 2004-158397, an ultrafine metal powder slurry having superior dispersibility can be provided; however, due to advanced quality requirements for conductive pastes, improvement in properties of ultrafine metal powder slurry itself (in particular, dispersibility, dry film density, and the like) has been further achieved. That is, when the dry film density is decreased, contraction of electrode films caused by firing is increased. As a result, areas of the electrode films are decreased or are partly broken off, and an ideal electric capacity may not be obtained due to a decrease in the effective electrode area (covering area). The decrease in effective electrode area may also cause a decrease in yield of the products. Recent technical developments of multilayer ceramic capacitors primarily aim at increasing the electric capacity. In order to achieve higher electric capacity, a technique of decreasing the thickness of electrode films is required. When the thickness of electrode films is decreased, the number of metal particles overlapping each other in one layer is decreased to 4 to 8 particles, which is approximately one third of the number of particles that have been used in one layer. Accordingly, by particles overlapping each other in the thickness direction of a multilayer film, an effective electrode area after firing has been ensured; however, when the thickness of electrode films is decreased, it becomes difficult to obtain the above effect. Accordingly, an increase in dry film density after coating, that is, increase in particle density must be achieved.